1. Technical Field
The present disclosure relates to an apparatus for generating extreme ultraviolet (EUV) light and a method for the same.
2. Related Art
A lithography apparatus used for fabrication of an integrated circuit or the like is an apparatus for transferring a desired pattern onto a substrate. A patterning device called a mask or a reticle is used for producing a circuit pattern on a substrate. Transfer of such a pattern onto the substrate is attained by imaging on a radiation-sensitive material (resist) layer provided on the substrate (for example, a silicon wafer substrate).
A critical dimension (CD) that is a theoretically predicted limit value of pattern transfer is given by the following formula (1).CD=K1·λ/NA  (1)Here, λ is a wavelength of light for light exposure used for pattern transfer, NA is a numerical aperture of a projection system used for the pattern transfer, and K1 is a process-dependent coefficient called a Rayleigh constant. CD is a critical dimension of a print. As can be seen from formula (1), a decrease of a transferable size can be achieved by any of the three, that is, a decrease of the wavelength λ of light for light exposure, an increase of the numerical aperture NA, or a decrease of the K1 value.
It has been proposed that an apparatus for generating EUV light with a wavelength in a range of 10 nm to 20 nm, preferably, in a range of 13 nm to 14 nm, is used in order to reduce a wavelength of light for light exposure and thereby reduce a transferable size. For a typical EUV light generation apparatus, there can be provided a laser produced plasma (LPP) type EUV light generation apparatus, a discharge plasma type EUV light generation apparatus, an electron storage ring generated synchrotron radiation type EUV light generation apparatus, or the like.
Typically, a tin (Sn) droplet is irradiated with a laser light beam to produce plasma and thereby generate light with a wavelength in a range of EUV in an LPP type EUV light generation apparatus. Such a laser beam may be supplied by, for example, a CO2 laser apparatus.